Laminate Storage and Handling Guide

A laminate and a prepreg are reactive materials. The resin in a prepreg is partly cured and continues to advance slowly at room temperature, and both the prepreg and the finished laminate absorb moisture from the air. Laminate storage is therefore a control on two clocks, the shelf life of the resin and the moisture content of the material, and a store that ignores either of them will produce boards that fail in ways the process cannot explain.

Two Clocks Running at Once

The first clock is the resin. A prepreg is supplied in a B stage, part way between a liquid and a fully cured solid, and the degree of cure advances with time and temperature. A prepreg that has advanced too far will not flow during lamination, so the resin stays where it was and the cavities around the copper are left empty.

The second clock is moisture. The resin absorbs water from the air, and the water turns to steam during lamination or during reflow. The result is a void in the laminate or a blister after assembly.

The two interact, because a warm store accelerates both. Cold storage slows the resin and lowers the moisture pickup, which is why most prepreg is shipped and kept refrigerated.

Refrigerated Storage

The usual condition is a temperature between about five and twenty three degrees with a controlled humidity, and the supplier datasheet gives the range for the specific material. The store should record the temperature continuously.

A refrigerator that is opened repeatedly has a temperature that cycles, and the material near the door sees a different condition from the material at the back. The store should be loaded so that the material that is used first is the oldest and the most accessible.

A domestic refrigerator is not suitable. It is too cold at the back, the temperature varies widely and there is no humidity control. The store should be a dedicated cabinet with a recording device.

Shelf Life

The shelf life quoted by the supplier applies to the storage condition that the supplier specifies. A prepreg that has been kept cold has a longer working life, while one that has been left warm has consumed part of its life without being used.

The figure should be recorded at receipt and the material should be rotated so that the oldest is used first. A store that has no rotation accumulates material that is at the end of its life, and the first sign is a lamination defect.

A material that is past its stated life is not automatically unusable, but it should be requalified. The test is a flow measurement and a lamination trial on a sample, and the result decides whether the material is released or scrapped.

Thawing and Condensation

A prepreg taken from a cold store will condense moisture on its surface when the packaging is opened in a warm room. The condensation has to be avoided, which means the material should be brought to room temperature inside its sealed packaging.

The warm up time depends on the roll size and on the temperature difference. A large roll can take many hours, and the material should be left for the full period rather than judged by the surface temperature.

Once the packaging is opened, the material should be used within the working life. The remainder should be resealed and returned to the store with the time out of the store recorded.

Packaging and Its Integrity

Prepreg is supplied in a moisture barrier bag with a desiccant and often with a humidity indicator card. A card that has changed colour is evidence that the packaging has been compromised and the material has absorbed moisture.

The bag should be opened with a clean cut and resealed with a proper seal, not folded and taped. A bag that is not sealed allows the material to equilibrate with the room, and the shelf life clock runs even though the material is back in the store.

The material should be supported during handling. A roll that is dropped or bent is damaged even if it looks intact, and the damage appears as a variation in the resin distribution across the panel.

Handling on the Production Floor

Gloves should be worn. Skin contact transfers oils to the surface, and the oil affects the bond between the prepreg and the copper, which appears as a small delamination after lamination.

The material should be cut in a clean area with a sharp tool. A torn edge carries loose glass that contaminates the surface and creates a defect that is difficult to trace.

The time a stack is left open before pressing should be limited and recorded. A laid up stack that sits for a shift absorbs moisture and may have begun to advance.

Finished Laminate Storage

A finished panel also has a shelf life, because the copper surface oxidises and the resin continues to absorb moisture. The oxidation affects the solderability and the lamination of a subsequent layer.

The panel should be stored in a dry place, on a rack, with the surfaces protected. Panels should not be stacked on an edge where the corners can deform, and they should not be stored where a heavy object can rest on them.

Where a panel is stored for a long period, the surface finish determines the shelf life. An organic coating has the shortest life, and an immersion finish or a plated finish lasts longer. The storage time should be recorded against the panel.

Records and Traceability

The record should show the material type, the batch, the date of receipt, the storage location and the date the packaging was opened. Those five items allow a defect to be traced to a specific roll or panel.

The temperature and humidity of the store should be recorded continuously and reviewed. A chart that shows a rise over a weekend is the explanation for the boards that failed on Monday.

Where the material is returned to the store after use, the time out should be added to the history. A roll that has been out for several short periods is not the same as one that has been out continuously, and the record should distinguish them.

Practical Rules

Keep the material cold and dry, thaw it in its packaging, rotate the stock and record every movement. Requalify a material that is past its life rather than assuming it is still good.

Record the storage data with the build records and the shelf life control, and review the moisture control rules and the resin content control when a lamination defect appears.

FAQ

Why must prepreg be thawed in its packaging? Opening a cold bag in a warm room condenses moisture on the material. Thawing inside the sealed bag avoids the condensation.

What happens if prepreg is stored warm? The resin advances toward its cured state and the material absorbs moisture. It will not flow properly during lamination and may blister later.

What should be recorded? The material type, the batch, the receipt date, the storage location, the date the bag was opened and the time out of the store, so a defect can be traced to a specific roll.